Eddie Murphy plays the raunchy black comedian Rudy Ray Moore, in a fairly accurate (afaict) depiction of his rise to fame. He paved the way for Richard Pryor, George Carlin and even Mr. Murphy himself by breaking down the taboos about race and sex and dirty words. Well, Redd Foxx did a lot of that too, well before Moore.... but he's mentioned only in passing. And Rusty Warren... but nevermind.
Moore's act was intentionally tasteless during a time when Deep Throat was in theaters. Moore's act -- and it was just an act -- was offensive during the time when everyone was afraid to utter the 'n' word and Bill Cosby was trying to provide a good role model for black kids. Moore was the black Chuck Barris, an entrepreneur / self-promoter / show-biz jack-of-all-trades, who promoted himself into a nationwide, but minor, fame.
As is to be expected, Murphy pulls out all the stops.
Like Jim Carrey playing Andy Kaufman, it's sometimes hard to define where the actor stops and the role begins. It's his first movie in 3 years, and his first R-rated (non-kids) movie since 1999.